Are you using a Delco internal regulator alternator ? If so, check if the #1 excite wire is not shorted to the #2 sense wire.

If the #1 wire is shorted to the #2 wire, #2 is hot all the time and will backfeed through the idiot light to the ignition coil if both the idiot light and coil are connected to the same ign switch terminal. This will make the light burn when the tractor is stopped.
If not already installed, buy a delco alternator harness repair plug at the auto parts store.
This plug makes for a more professional liiking job and is safe from shorts.

If the gen light goes out when you unplug the #1-#2 plug at the alternator, that would point to an internal short in the alternator or regulator.
Delco alt wiring.

Sorry I misunderstood what you were saying, I thought that you had a generator with an external Regulator. We don't get many tractors with an Alternator and a warning light they usually fit an ammeter as it gives more information. So apologies for the misunderstanding...
John(UK)..fergusontractors@hotmail.com

(quoted from post at 09:10:11 08/06/12) Sorry I misunderstood what you were saying, I thought that you had a generator with an external Regulator. We don't get many tractors with an Alternator and a warning light they usually fit an ammeter as it gives more information. So apologies for the misunderstanding...
John(UK)..fergusontractors@hotmail.com

don't be sorry John,I appreciate all information given,haven't had time to look at tractor since i posted question,it does seem strange that this problem wasn't happening with the old alternator,thanks for everyones reply.

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